My apologies for the late reply, but I was waiting for you to update your status. I understand you have already purchased the amp.

If you had not already purchased the amp, I would say the first thing would be to figure out what version it is. The best thing to do is to take a well lit pic of the bottom of the amp. With the bottom off of course. The photo should be straight on, and reasonably detailed. If you send me the pic, I can tell you everything there is to know about this amp.

Since you have requested upgrade info, please send me the pic above, and a brief list of your desired results.

Well, just purchased this amp and thought it sounded ok without being special.

After a lot of reading about the issues I thought I'd have a look inside and see what I've got. Found the bias for all tubes had been would right back to the minimum. Reset them all to 600mv (no load) without any issues. Did a bit of wiring housekeeping and put the cover back on.

Turned it on once back in the system and WOW, this is I think what it should sound like. A different amp.

All was good for about 15-20 mins then one channel turned bad real quickly, opened my eyes to see all tubes on one channel glowing like a light bulb! Jumped up and turned it off. Waited for it to cool then turned it on again, sweet sound once again, I didn't wait for it to go into thermal runaway again.

So what do we need to fix the thermal runaway first? Once we address this i'll look to do the reliability mods as directed by Jim.

First, the tubes that have run away are now junk. They may not test that way on a tester, but these amps run a 6L6GC harder than they should be run. If you bias too low, the amp will lose all of its magic. For the fast and dirty bias, powering up the amp, allow 3 minutes for warm up. You will begin the bias process rotating the pots counter-clockwise as necessary for increased bias and vice-versa. Bias 610-615mV. Only allow a seven minute window of adjustment. If the unit is on longer than 10 minutes collectively, shut it off until cool to the touch (about 30 mins.). Try again.

Before doing this, R43 will need to be replaced. It is one of the cathode resistors. You can see it discolored in the photo. That is one of the parts in any of our kits. 5% of the time, the output trans will be lost before the factory resistor gives up. You do not want that. Further, it is not uncommon for them to ignite the circuit board. The numbers you are reading while biasing, are the voltage drop across the cathode resistor. So you have no idea what you are actually biasing V6 to, and the tubes on that side of the amp are heat distressing the output transformer. Personally, I would not continue to run the amp.

All of those electrolytic capacitors are at least 20 years old. The Cathode resistors are over sized. Other resistors are either under sized, over voltaged, or the wrong value. At least two resistors are transposed, and one trace error exists. D9 is a ticking time bomb, and C7, C26, C46, or C48 are likely over voltaged every time the amp is started due to no centering resistors or power supply regulation. The amp is hard to bias with 270 degrees of carbon film on a plastic substrate (the bias pot). Around $150 worth of parts, and a new set of tubes is my recommendation.

The SE40 can sound close to, if not magical when the issues with them are fixed. Smoked some cathode resistors in mine before I figured out that there were congenital defects with the amps in every revision they made.